The present invention relates to a photoelectric position measuring instrument comprising an illumination unit, a scale grid, a scanning grid defining at least two scanning fields and optical means allocated to the scanning fields wherein the scale grid and the scanning grid operate as phase grids to produce, in cooperation with the illumination unit and the optical means, a first and second set of diffracted partial light rays of substantially opposite relative phase.
Position measuring instruments using phase grids are known. Examples of such measuring instruments are disclosed in German published application DE-AS No. 15 48 707 and German unexamined patent specification DE-OS No. 23 16 248.
Additionally, in the non-prior-published European patent application No. 85301077.5, corresponding to EP No. 0 163 362 A1, a position measuring instrument is disclosed wherein light rays, which are diffracted on an index grid, are generated with relative phase displacement. The index grid comprises a plurality of slats and furrows and is constructed as a phase grid. The phase displacement is produced by means that, for a given grid constant, the width of a slat of the index grid differs from the width of a furrow.
Phase grids of this type produce light rays of different diffraction orders. The orders may be displaced with respect to one another and used to determine the direction of the relative displacement between the scale grid and the index grid.
In a direct light, phase grid measuring system, the degree of modulation of the zero and .+-. first order diffraction signals are different from one another and may vary. The index and scale grids of such a system are arranged parallel to one another and comprise slats and furrows of unequal width. The degree of modulation may vary with changes in the distance between the index and scale grids, dirtying of the grids, and the like.
The variation in the degree of modulation occurs in accordance with the three grid scanning principle and is attributable to the unavoidable geometric optical imaging of the light source onto the photodetector used for detecting the zero diffraction order light rays. The three grid scanning principle is illustrated in U.S. Pat. No. 4,051,367.
A disadvantage of the above described system is that the variation in the degree of modulation results in an inaccurate and uncertain electronic evaluation of the signals.
It is an object of the present invention to reduce, in position measuring instruments of this type, the measuring sensitivity with respect to distance changes, deviations from parallelism of the grids, fouling, and the like.